The preparation of oxazolidones from an organic isocyanate and an epoxide using a quarternary ammonium halide and an alcohol as cocatalysts is disclosed in U.S. Pat. No. 3,313,747. It is also known that zinc bromide catalyzes the foregoing reaction, Sandler, J. Polymer Science A-1, 5, 1481 (1967). Lithium chloride has been found by K. Gulbins to be a catalyst for the reaction between an aromatic isocyanate and an aromatic epoxide. (Chem. Ber. 93, 1875, 1960). n-Butoxy lithium has been found by R. R. Dileone to be an oxazolidone catalyst (J. Polymer Science, Part A-1, Vol. 8, p.609, 1970). An adduct of lithium bromide and tributyl phosphine oxide has been reported by J. E. Herweh and W. J. Kauffman to be an oxazolidone catalyst (Tetrahedron Letters, No.12, p,809, 1971). U.S. Pat. No. 3,702,839 (Nov. 14, 1972) disclosed that a phosphonium salt was an oxazolidone catalyst. S. Kimura and H. Samejima (Japanese Patent Public Disclosure 48-70797, Sept.,25, 1973) disclosed that monomer-soluble catalyst selected from the group consisting of specific organic metal chelates alkyl glycidyl ammonium salts, trialkyl (triaryl or tricycloalkyl)borates, and trialkyl (or triaryl)phosphines are oxazolidone catalysts. Y. Iseda, F. Odaka et al. (Japanese Patent Public Disclosure 49-37999, Apr., 9, 1974) have disclosed that epoxy-soluble quaternary ammonium compounds and/or lithium halide-phosphine oxide complexes are oxazolidone catalysts. U.S. Pat. No. 3,817,938 (June 18, 1974) disclosed that alkoxide or phenoxide of a metals of Group IIA or IIIA of the Periodic Table was an oxazolidone catalyst. It has now been found that the hereinbelow disclosed and claimed dialkyl zinc, zinc carboxylate, organozinc chelate compound and trialkyl aluminum are much more effective catalysts for the preparation of oxazolidones.